Determination of PD-L1 expression in effusions from mesothelioma by immuno-cytochemical staining.

BACKGROUNDMalignant mesothelioma (MM) is an aggressive, fatal tumor. Current therapeutic options only marginally improve survival. Programmed cell death ligand 1 (PD-L1) is a dominant mediator of immunosuppression, binding to programmed cell death 1 (PD-1). PD-L1 is up-regulated in cancer cells, and the PD-1/PD-L1 pathway plays a critical role in tumor immune evasion, thus providing a target for antitumor therapy. Further, a correlation between PD-L1 expression and prognosis has been reported. Studies performed on histological material have revealed expression of PD-L1 in MM, but no study has been performed on MM effusions thus far. METHODSPD-L1 expression was determined by a commercially available antibody (clone 28-8) in 74 formalin-fixed, paraffin-embedded cell blocks from body effusions obtained at diagnosis from patients with MM. The presence of MM cells was confirmed with CK5/6, calretinin, and EMA and the admixture of macrophages was assessed with CD68. Only cases containing more than 100 tumor cells were assessed. Membranous staining in tumor cells was considered positive. Survival time was calculated from the appearance of the first malignant effusion until death. RESULTSReactivity was observed in 23 of 61 (38%) of cases and was classified as 1%-5% (n = 9 cases), >5%-10% (n = 4 cases), >10%-50% (n = 4 cases), and >50% (n = 6 cases) positive cells. Survival times did not differ significantly between patients with PD-L1-positive and PD-L1-negative tumors. CONCLUSIONMM effusions are suitable for immune-cytochemical assessment of PD-L1 expression in malignant cells and the results are similar to those reported for histological specimens. Cancer Cytopathol 2017;125:908-17. (c) 2017 American Cancer Society. We studied the PD-L1 reactivity of tumor cells in 74 formalin-fixed, paraffin-embedded cell blocks from body effusions obtained at diagnosis from 74 patients with malignant mesothelioma. In 61 cases, the PD-L1 reactivity could be soundly determined, and the number of positive cases agrees with the outcomes of other studies performed on histological material, suggesting the determination of PD-L1 status in effusions as an early and convenient alternative.